In the design of EAS systems which use magnetic type markers, efforts have been made to enhance the uniqueness of the marker's response. One way that this has been accomplished is by increasing the high harmonic content in the voltage pulse generated by the magnetic flux reversal of the marker. As a result, the marker's response signal becomes more easily differentiated and detectable over the lower frequency background noise and magnetic shield noise and signals generated by other magnetic materials often found to exist in EAS systems.
A magnetic marker which exhibits a high degree of uniqueness is disclosed in U.S. Pat. No. 4,980,670, entitled "DEACTIVATABLE E.A.S. MARKER HAVING A STEP CHANGE IN FLUX", and assigned to the same assignee herein. In the marker of the '670 patent, the uniqueness of the response of the marker is attributable to the hysteresis characteristic of the material. In particular, the hysteresis characteristic exhibits step changes in flux at threshold values of the applied field.
The aforesaid hysteresis characteristic is brought about by conditioning the material of the marker so that it has a pinned domain wall configuration that remains pinned until the applied field reaches a predetermined threshold value, at which the pinned condition is overcome by the applied field causing a step change in flux. Such step change in flux provides a response signal from the marker which is rich in high harmonic content. The response signal is thus unique and, therefore, easily detectable.
The conditioning described in the '670 patent to achieve such pinned wall effect requires that the marker material be heated or annealed to pin the desired domain wall configuration. The '670 patent describes a number of soft amorphous transition metal-metalloid compositions (Co.sub.75.2 Fe.sub.4.8 Si.sub.2 B.sub.18 ; Co.sub.74.26 Fe.sub.4.74 Si.sub.3 B.sub.18 ; Co.sub.74.24 Fe.sub.4.76 Si.sub.2 B.sub.19) which can be used as the marker material. It also describes annealing temperature and time conditions.
When heating the soft (Co--Fe--Si--B) amorphous materials described in the '670 patent using certain of the described temperature and time conditions, the amorphous materials develop a crystallized inner layer of ferromagnetic material depleted of much of its glass forming metalloid and comprised substantially of crystallized Co with some Fe dissolved. This layer surrounds a bulk portion of material which comprises the original soft metal-metalloid composition. The resultant annealed material contains the desired pinned wall configuration and provides the step flux effect. In addition to the crystallized layer an outer surface layer of oxide is formed (e.g., SiO.sub.2 and B.sub.2 O.sub.3, i.e., a borosilicate). However, the oxide layer is non-magnetic.
Deactivation of the '670 marker is accomplished by exposing the marker to a high frequency or high amplitude AC magnetic field. In each case, additional domain walls are created in the marker and these walls remain in the marker after the marker is removed from the deactivating field. The presence of these additional domain walls prevents the original flux step from reoccurring. Moreover, once deactivated, the '670 marker cannot be easily reactivated. This prevents the marker from being reusable.
There are instances where it is advantageous to have markers which are capable of being deactivatable and also reactivatable.
It is therefore a primary object of the invention to provide an improved deactivatable marker.
It is another object of the present invention to provide an improved marker that it is reactivatable in addition to being deactivatable.
It is a further object of the present invention to provide a method of making an improved marker.
It is a further object of the present invention to provide an electronic article surveillance system incorporating the aforementioned improved magnetic marker.
It is yet a further object of the present invention to provide an electronic article surveillance system incorporating deactivation means, reactivation means, and the aforementioned improved magnetic marker.